According to known processes, the membrane serving as a guarantee seal is either produced by deep drawing with a cylindrical edge fixed directly to the inner wall of the can be sticking or heat-sealing (DAS 2,061,497) or the edge of the foil is stuck to a fastening ring made from sheet metal or plastic and enclosing the edge of the can, said ring serving as the edge protection on the can opening edge (German Pat. No. 1,814,951). According to this prior art, on opening the can the membrane is detached along the adhesive joint by pulling on a pull-off tab. These known can closures have the disadvantage of being costly and complicated to manufacture, and after opening the can, the edge thereof is irregular due to adhesive residue, and more particularly the inwardly projecting portion of the fastening ring is a hindrance on emptying the can.
In order to prevent, in the known construction with a cylindrical membrane edge, the destruction of the edge protection on the can opening formed by part of the membrane on tearing away the membrane, a separating cut is provided in the stuck can edge which extends both through the edge protection and through the inner lining of the can into the cardboard can body. Due to the limited precision, it is not possible to reliably make such a shallow cut with a cutting tool and in addition it is technically far from easy to make a cut on the inner wall of the can which runs in the peripheral direction of the can. An important disadvantage of this known can closure is also that on removing the membrane, part of the inner lining of the can is also usually torn away, so that the cardboard material comes into direct contact with the product contained in the can giving an unhygienic impression. When using a thicker foil material, i.e., sheet metal, in which according to the prior art (DAS 1,757,311) a groove is stamped therein without cutting the membrane, it is no longer possible to fix the membrane in the can through sticking due to the fact that the foil material is not easily deformable, so that such sheet metal closures can only be fixed to the can by flanging around the can edge. It is not possible in this way to fix metal foils with a thickness less than 150.mu. (0.15 mm.) because then the can edge can no longer be embraced in clamping manner by the flange or the clamping forces cannot be taken up by the thin foil material.
Furthermore, a can closure is known which has a closure membrane whose thickness is less than 150.mu. (0.15 mm.) and in which the desired separating line is located in a portion of the membrane which covers the can opening. In this case the membrane is already cut along the desired separating line by means of a punch and the cut line is covered by a thin foil stuck to the back of the membrane.